An organic electroluminescent (OEL) device, alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; it is operable with a low driving voltage of the order of a few volts and clearly viewable from oblique angles. These unique attributes are derived from a basic OLED structure comprising of a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al in commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 have disclosed such a structure. The common electroluminescent (EL) medium is comprised of a bilayer structure of a hole-transport (HTL) layer and an electron-transport layer (ETL), typically of the order of a few tens of nanometer (nm) thick for each layer. When an electrical potential difference is applied at the electrodes, the injected carriers—hole at the anode and electron at the cathode—migrate towards each other through the EL medium and a fraction of them recombines in the emitting layer (ETL) a region close to the HTL/ETL interface, to emit light. The intensity of electroluminescence is dependent on the EL medium, drive voltage, and charge injecting nature of the electrodes. The light viewable outside of the device is further dependent on the design of the organic stack and optical properties of the substrate, anode and the cathode.
Conventional OLEDs are bottom emitting (BE), meaning that the display is viewed through the substrate that supports the OLED structure. The devices normally employ glass substrates having a layer of highly transparent indium-tin-oxide (ITO) layer that also serves as the anode. The cathode is typically a reflective thin film of MgAg although lithium-containing alloys are also used as an efficient electron injecting electrode. The light generated within the device is emitted in all directions. However, only a small fraction of generated light is available for viewing, and about 80% of generated light is trapped within the device in waveguiding modes in glass, ITO and organic layers. The light emitted toward the anode at less than the critical angle passes through the anode and the substrate to the viewer, and the light emitted in the opposite direction is reflected at the cathode and passes through the substrate, enhancing the viewing intensity. A high-transparency substrate and anode and a high-reflectivity cathode are thus preferred.
The OLED display is typically coupled with active matrix (AM) circuitry in order to produce high performance displays. For the AM bottom emitting display, which uses switching elements of thin film transistors, the transistors are fabricated on glass substrates. Consequently the open area available for the light to emerge is reduced. With the application of multi-transistor and complex circuitry in the backplane the open area through which the light emerges is reduced. The ratio of the open area to that of the entire display area is called the aperture ratio. Due to the reduction of the aperture ratio the display will run dim. To compensate for the reduced average brightness level, the drive current has to be increased subjecting the display to increased risk of operational degradation. It follows that more complex pixel drive circuitry cannot be readily implemented without further compromising aperture ratio and operation stability.
To alleviate this problem the emitted light can be made to emerge through top surface. In the top-emitting design the drive circuitry is fabricated on substrate and the light emits from the opposite surface. This design permits the use of complex circuitry occupying whatever substrate space is needed and the aperture ratio is not affected. The high aperture ratio makes the display viewable consuming less power. The devices have the prospect of running at low drive current while maintaining readability and thus extending the operational life.
The devices employing opaque backplanes such as silicon the OLED must be of the top-emitting type. The top surface, usually the cathode, needs to be at least semitransparent in order to allow the light to exit through the top. The device should preferably include a reflector or a reflecting anode opposite to the cathode side to redirect the light emitting toward the anode.
Any device design top- or bottom-emitting should be aimed at achieving highest possible efficiency. However, realizing high efficiency by reclaiming light lost to waveguiding modes can be very difficult. To recover even a fraction of light lost to waveguiding modes the device architecture can be very complex.
An approach to enhance the efficiency without introducing such complexity is to implement the microcavity design of the structure of the device which includes reflecting electrodes. Sony Corporation (EP 1 154 676 A1) has disclosed an anode made of light-reflecting materials such as Pt, Au, Cr, W, or presumably other high-work function materials in conjunction with an optional buffer/hole-injecting layer (HIL). Sony also reported (EP 1 102 317 A2) an anode composed of a transparent conducting film such as ITO formed on the reflecting layer. The top electrode was a semitransparent reflecting layer of MgAg or Al:Li alloy serving as the cathode through which the light emerges. Lu et al. reported top-emitting, highly efficient OLEDs that used reflective metals in the anode structure, a phosphorescent emissive layer, Ir(ppy)3, and a semitransparent compound cathode. (“High-efficiency top-emitting organic light-emitting devices”, M.-H. Lu, M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M. Hack, and J. J. Brown, Appl. Phys. Lett. 81, 3921 (2002)). Riel et al. demonstrated (“Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling”, H. Riel, S. Karg, T. Beierlein, B. Rushtaller, and W. Rieb, Appl. Phys. Lett. 82, 466 (2003) high-efficiency top emitter, also using the Ir(ppy)3 emissive layer, high work-function metal anodes, and semitransparent cathodes and further employing a ZnSe layer over the semitransparent compound cathode for improved light outcoupling. These top-emitters demonstrated efficiencies that are higher than the equivalent bottom-emitting non-microcavity devices. Raychaudhuri et.al. reported top- and bottom-emitting microcavity devices that are twice as efficient as the optimized bottom-emitting non-microcavity device (“Performance enhancement of bottom-and top-emitting organic light-emitting devices using microcavity structures”, P. K. Raychaudhuri*, J. K. Madathil, Joel D. Shore and Steven A. Van Slyke, Procceedings of the 23 rd International Display Research Conference, Phoenix, Ariz., Sep. 16 to 18, 2003 p 10).
By employing highly reflective electrodes it is possible to remarkably increase the out coupling of generated light. In the microcavity device the light emitted out of the cavity depends on the cavity design. The resonance wavelength of a microcavity is given by2Σ(nidi)/λ−(Φ1+Φ2)/360°=m,where m=0, 1, 2, . . . , λ is the peak wavelength of the light emerging out of the cavity consisting of layers having the thickness di and refractive index ni, and Φ1 and Φ2 are the phase shifts of light in degrees upon reflection from the two reflecting electrodes. The quantity nidi is conventionally called the “optical path length” in the material, so Σ(nidi) is the total optical path length in the microcavity. For a fixed optical path length of the microcavity, the strength of this emission (and to a much lesser degree its wavelength) is also influenced by the location of the emission zone within the cavity. If a reasonable distance between the cathode and HTL/ETL interface is selected, we can then vary the HTL thickness in order to vary the optical path length of the microcavity. The maximum luminance occurs for an HTL thickness such that the resonance wavelength of the microcavity is well-aligned with the peak in the innate emission spectrum of the particular dopant-host material. The first maximum occurs at the HTL thickness corresponding to m=0, and subsequent maxima occur at the HTL thickness corresponding to m=1, 2 and so on.
FIG. 1 is a plot of luminance of a top-emitting microcavity structure as a function of NPB HTL thickness up to the thickness corresponding to m=1 as determined by optical modeling. The structure of the OLED is: Glass/80 nm Ag NPB (variable)/60 nm Alq/14 nm MgAg which includes the fully reflective Ag anode deposited on the glass substrate. The MgAg top electrode is the cathode, and being thin and semitransparent allows significant emission through the top surface of the OLED.
FIG. 1 shows that for this structure the first maximum of the luminance occurs at 46 nm and the second maximum occurs at 196 nm of NPB thickness. It is obvious that plots of the luminance as a function of NPB HTL thickness for the emission at other wavelengths will be similar to the FIG. 1 but the cavity length will be slightly different, and the maxima of resonance wavelengths will occur at slightly different NPB thicknesses. It has been found, however, that the OLED based on the above layer structure was inefficient. An interlayer between the Ag and NPB HTL was found useful in lowering the operating voltage and improving the stability of the OLED. The interlayer, which functions as the hole-injection and diffusion barrier, is very thin (1 to 2 nm thick) and highly transparent. Thus it does not significantly affect the optics of the OLED. The OLED structure with 46 nm NPB is most desirable for maximizing efficiency and minimizing the angular dependence of the color but the one with the 196 nm NPB can be desirable from a manufacturing point of view. This is because the thin NPB can yield shorted OLEDs that can catastrophically fail in use. On the other hand, thick NPB HTL is likely to increase the drive voltage.
FIG. 2 is a plot of luminance of a bottom-emitting microcavity structure as a function of NPB HTL thickness up to the thickness corresponding to m=1 as determined by optical modeling. The structure of the OLED is: Glass/20 nm Ag/NPB (variable)/60 nm Alq/200 nm MgAg which includes the reflective, semitransparent and conductive Ag anode deposited on the glass substrate. The MgAg top electrode is the reflective, opaque and conductive. The Ag anode being thin and semitransparent allows significant emission through substrate.
Further, FIG. 2 is very similar to FIG. 1 and shows that for this structure the first maximum of the luminance occurs at about 50 nm and the second maximum occurs at about 200 nm of NPB thickness. It is obvious that plots of the luminance as a function of NPB HTL thickness for the emission at other wavelengths will be similar to the FIG. 1 but the cavity length will be slightly different, and the maxima of resonance wavelengths will occur at slightly different NPB thicknesses. Here again, the OLED structure with 50 nm NPB is most desirable for maximizing efficiency and minimizing the angular dependence of the color but the one with the 200 nm NPB can be desirable from a manufacturing point of view and stability. In the bottom-emitting device a thin hole injecting layer (HIL) including a fluorinated carbon or an oxide, disposed between the Ag anode and the NPB HTL was found to reduce the drive voltage and improve the efficiency. The HIL is very thin (1 to 2 nm thick) and highly transparent, and does not significantly affect the optics of the OLED. The HIL can also act as a diffusion barrier and improve the stability of the diode.
In operation, OLEDs in general, degrade in performance characterized by loss of luminance with concurrent increase in drive voltage. This may indicate changes in the bulk of the active layer as well as degradation of injecting contacts of the diode. An active matrix OLED display is operated at constant current requiring continuous increase in drive voltage during operation to maintain the drive current. The provision is made in the drive circuits of the AM OLED backplane to provide the excess voltage on demand. This provision results in wasted power even if the excess voltage is never used. If a display device is made that would not require adjustment in drive voltage in operation, then the drive circuit can be designed with little or no voltage allowance. This would result in substantial saving in power consumption. Such devices are those with contacts that would not degrade in extended use.